Lost 2nd monitor in KDE upgrading to 22.04 from 20.04

[gauss]

I used KDE 20.04 and my two monitors merged normally. I did a do-release-upgrade and now my second monitor displays correctly (merged) until the KDE icon disappears and KDE launches. At that point it goes blank. I did a fresh Ubuntu 22.04.03 install, preserving /home and then installed kde-full. Still KDE (Plasma X11 from SDDM) has the same problem with the second monitor. The first monitor displays normally. When I boot Ubuntu (GNOME) from SDDM both monitors function correctly, merged. When I boot KDE from a live USB KDE linstall ISO, both monitors are normal, merged.

Any debugging hints (e.g. logs to consult) will be gratefully received.

[SeijiSensei]

Have you tried using System Settings > Display to resolve the problem? You can also get there by right-clicking on the Desktop and choosing "Configure Display."

[gauss]

Have you tried using System Settings > Display to resolve the problem? You can also get there by right-clicking on the Desktop and choosing "Configure Display."

That was it. Both monitors appeared but the left one had 60 Hz refresh rate and the right one 75 Hz. When I changed the right one to 60 Hz, the right monitor displayed normally, with both monitors creating a merged desktop.

Many thanks.